In order to reduce the blur of writing line, attempts have been made to prevent spreading of ink to a paper space absent of writing line from the writing line by adjusting the surface tension of ink or the amount added of a penetrating component and thereby suppressing penetration of ink into paper or by adjusting the rheology of ink and thereby suppressing the flowability of ink on the writing line.
As for the technique to reduce blur of writing line by adjusting the rheology of ink, it is known to suppress the flowability of ink by making the ink viscosity high or increase the storage modulus. Also, when the ink viscosity is made high, flowability in writing line is usually lost to cause ink depletion in the writing line, and therefore, there is known a method for obtaining clear writing line by imparting a shear viscosity reduction to the ink and despite high viscosity during standing still of the ink, reducing the viscosity by a shear force attributable to rotation of a ball during writing, thereby preventing both blur of writing line and occurrence of ink depletion. For example, it is known in Patent Document 1 that in an oil-based ink for a pressurized ballpoint pen, the viscosity is 50,000 mPa·s or more at a shear rate of 0.19 (20° C.) and becomes 30,000 mPa·s or less at a shear rate of 500 s−1 (20° C.). Furthermore, Patent Document 2 discloses an oil-based ink composition where the viscosity at a shear rate of 1 s−1 is adjusted to from 10,000 to 500,000 mPa·s by adding a shear viscosity reduction-imparting agent.
Patent Document 3 discloses an ink composition where the “thickening” of the line width protruding outside the printing region due to flowing of an electrically conductive ink is eliminated by adjusting the composition to have a storage modulus of 5,000 to 50,000 Pa and tan δ≦1 at a frequency of 1 Hz and a vibration stress of 50 Pa.
On the other hand, in order to reduce the ink depletion in writing line, attempts have been made to uniformly apply ink to the ball surface by adjusting the surface tension of ink or the amount added of a penetrating component and thereby enhancing the wettability of ink to the ball surface or by adjusting the rheology of ink and thereby increasing the flowability of ink on the ball surface. As for the technique to reduce ink depletion in writing line by adjusting the rheology of ink, it is known to increase the flowability of ink by making the ink viscosity low or adjust tan δ of ink. Patent Document 3 also discloses an ink composition where tan δ at 100 rad/s is set to 1.0 or less and the modulus response is thereby maintained even when the ink becomes flowable because of reduction in viscosity due to shear during writing, as a result, the ink hardly splits and is less likely to be pulled aside and in turn, ink depletion in writing line is eliminated.
As for the rheology adjusting agent to impart such rheology characteristics to ink, a higher fatty acid soap, an oil/fat derivative, a silica particle, an acrylic resin, a cellulose derivative and the like are known as a material for imparting a shear viscosity reduction to ink. Among others, in Patent Documents 5, 6 and 7, specific examples using a cellulose derivative are disclosed as a material capable of imparting a large shear viscosity reduction to ink with a small addition amount.